Parasitology. Alan Gunn

leaves long‐lasting sores, and a further 500,000 with the potentially fatal visceral leishmaniasis of whom up to 80,000 die every year. The incidence of HIV/Leishmania co‐infections is also increasing and a serious cause for concern.

      4.2.1.1 Leishmania Life Cycle

In the vertebrate host, Leishmania exists in the amastigote form (2.5–5 μm) within mononuclear phagocytes and in particular the macrophages (Figure 4.3). These are a sub‐group of the leukocytes (white blood cells) that have an essential role in the immune response in which they phagocytose and destroy foreign organisms. The parasites multiply by binary fission within a phagocyte until the host cell is destroyed after which they are released to be ingested by, and subsequently invade, new phagocytes (Figure 4.4). Transmission is normally by female sandfly vectors: only the female sandfly feeds on blood – the males (Figure 4.5) are harmless nectar feeders. Parenteral transmission, for example via contaminated needles or blood transfusion is also possible. Venereal and transplacental transmission of L. infantum sometimes occurs in dogs (Magro et al. 2017), but at the time of writing, there was little information on whether it also occurs in other species of Leishmania or in hosts other than dogs.

Figure 4.3 Spleen smear showing Leishmania donovani amastigotes infecting macrophages. Many of the infected cells have burst and released the parasites. Owing to the small size of the amastigotes, it is extremely difficult to distinguish between Leishmania species using a light microscope.

Figure 4.4 Generalized life cycle of Leishmania spp. The parasite exists in the promastigote form (PM) in the sandfly vector and the amastigote form (AM) in the mammalian host. Some Leishmania species infecting humans are zoonotic and infect other mammals such as dogs and rodents. 1: An infected sandfly injects promastigotes when it feeds, and these are taken up by macrophages and other mononuclear phagocytes. Within these cells, the parasite is enclosed within a phagosome. Here, it transforms into the amastigote form and reproduces asexually. Eventually, the host cell is destroyed, and this releases amastigotes that infect other cells. A sandfly becomes infected when it ingests blood containing infected phagocytes. After reaching the sandfly midgut, the parasites transform to the promastigote stage, reproduce asexually and undergo several morphological changes and migrations within the gut. 2a: In species belonging to the subgenus Leishmania, the promastigotes move to the anterior midgut (amg) and then to the stomadeal valve (sv). 2b: In species belonging to the subgenus Viannia, the parasites first move the posterior midgut (pmg) and then forward to the stomadeal valve. In both subgenera, the parasites are ejected when the fly attempts to feed. For further details, see text. Drawings not to scale.

The sequence of development within the sandfly vector varies slightly between species, but in all cases involves transformation, replication, and subsequent movement to the anterior region of the gut. A sandfly acquires an infection when it ingests infected mononuclear phagocytes during feeding. Like mosquitoes, sandflies are ‘batch processors’ that take in a large blood meal that is then enclosed within a peritrophic membrane when it enters the midgut. They hold the blood meal within the midgut and digest it after which the products are absorbed. There is therefore usually a gap of several days between blood meals. When the blood meal first reaches the insect’s midgut, the amastigotes transform to the procyclic promastigote stage. This occurs in response to the decrease in temperature and increase in pH. The procyclic stage has a relatively short flagellum, is not very motile, and multiplies by binary fission within the blood meal. The promastigotes undergo a series of morphological transformations and multiplications as they move up from the midgut to the region of the stomodeal valve that marks the boundary between the foregut and the midgut. Along the way, some of the parasites attach to the lining of the gut and stomodeal valve using their flagellae, and this attachment is an important part of the life cycle. In those species belonging to the sub‐genus Viannia, most parasites make their way to the pylorus region (hind triangle) at the posterior of the midgut before moving forward. Unlike Plasmodium (in which the infective stages invade the salivary glands of the vector mosquito), the metacyclic stages of Leishmania do not, as a rule, infect the salivary glands of the sandflies. However, sandfly saliva does play an important role in the transmission process.

Figure 4.5 Lutzomyia longipalpis adult male; male sandflies have prominent claspers at the end of their abdomen. Sandflies are small (<4 mm in length), and their wings and body are covered in hairs. Their palps are very long and fold backwards. In common with many other nematoceran flies (e.g., mosquitoes), they have long legs, and their long antennae consist of many joints. Only female sandflies consume blood.

      How Harming the Vector Facilitates Transmission

      On reaching the stomodeal valve, the Leishmania promastigotes secrete a gel‐like substance called promastigote secretory gel, the main component of which is filamentous proteophosphoglycan, and some of them transform into the infective metacyclic promastigote stage (this has a long flagellum and is very active). The gel physically blocks the gut, and this together with the vast numbers of parasites severely compromises the fly’s ability to feed. Further compounding this, the parasites also produce chitinase enzymes that physically damage the peritrophic membrane and stomodeal valve. Because the insect’s ability to ingest food is impaired, it becomes hungry thereby increasing its probing and number of visits to hosts all of which increases the chance of transmission. Physical probing probably does not transfer many parasites but to ingest food the infected fly must first expel some of the promastigote secretory gel. This gel contains numerous infective metacyclic stage parasites, as well as other non‐infective stage(s). The secretory gel also facilitates the establishment of the infection in the vertebrate host, so it has a dual role in both the invertebrate and vertebrate host (Giraud et al. 2019).

      The transmission mechanism(s) employed by the Sauroleishmania remain uncertain. Within the sandfly vector, these species tend to remain in the posterior regions of the gut, and therefore, it is unlikely that transmission occurs when the sandfly feeds. Furthermore, sandflies do not usually defaecate whilst feeding, so it is unlikely that transmission resembles that of T. cruzi by triatomid bugs. A third possibility is that the transmission occurs through the lizards consuming infected sandflies.

Much of the work on how Leishmania establishes and develops within its mammalian hosts involves mice as model organisms and those few Leishmania species capable of being cultured in the laboratory. One should therefore be careful of extrapolating from these studies to the likely behaviour of other Leishmania species and infections in other hosts. Mice and humans are both mammals, but one cannot assume that their immune systems react identically to the same infectious agent. Furthermore, leishmaniasis manifests itself in numerous ways. Therefore, different Leishmania species and strains probably exhibit variations in the way they establish themselves and interact with the host immune system. Nevertheless, all species follow a basic pattern of development following their entry into the blood stream that involves morphological and physiological transformations and establishment in the mononuclear phagocytes and in particular the macrophages.